1. Field of the Invention
The present invention relates to a four-wheel steering vehicle, and more particularly to an electro-hydraulic control apparatus for a rear wheel steering mechanism of the vehicle which includes a hydraulic power cylinder arranged to be operated under control of fluid under pressure supplied thereto for effecting axial displacement of a relay rod operatively connected at its opposite ends to a set of dirigible rear road wheels.
2. Discussion of the Prior Art
In Japanese Patent Early Publication No. 199568/1987, there has been proposed such an electro-hydraulic control apparatus for a rear wheel steering mechanism as described above. The electro-hydraulic control apparatus includes a steering angle ratio control mechanism mechanically connected to a front wheel steering mechanism and electrically connected to an electric control apparatus and a directional control valve arranged to be operated under control of the steering angle ratio control mechanism for selectively connecting a fluid supply passage to one of opposite fluid chambers of the power cylinder and connecting a discharge passage to the other fluid chamber of the power cylinder. In operation of the directional control valve, the rear road wheels are steered in response to steerage of the front road wheels of the vehicle, and the steering angle ratio of the rear road wheels relative to the front road wheels is electrically controlled at the steering angle ratio control mechanism.
In the electro-hydraulic control apparatus, an electromagnetic flow control valve is disposed within the fluid supply passage and the discharge passage at the upstream of the directional control valve to provide a fluid communication between the passages in its deenergized condition and to interrupt the fluid communication between the passages in its energized condition, and a cut-off valve is disposed within the fluid supply passage and the discharge passage at the downstream of the directional control valve to be applied with a first pilot pressure from the fluid supply passage between the flow control valve and the directional control valve and with a second pilot pressure from the discharge passage between the flow control valve and the directional control valve. In operation, the cut-off valve acts to permit the flow of fluid passing through the fluid supply passage and discharge passage only when the first pilot pressure is maintained higher than the second pilot pressure. When the first pilot pressure becomes equal to the second pilot pressure in the occurrence of trouble of the fluid supply system, the cut-off valve acts to interrupt the fluid communication between the directional control valve and the power cylinder thereby to restrict steerage of the rear road wheels. In the occurrence of trouble of the electric control apparatus, the electromagnetic flow control valve is deenergized to provide the fluid communication between the fluid supply passage and the discharge passage thereby to forcibly equalize the first and second pilot pressures. Thus, the cut-off valve acts to interrupt the fluid communication between the directional control valve and the power cylinder for restriction of steerage of the rear road wheels.
In the electro-hydraulic control apparatus, however, the foregoing fail-safe function may not be effected in the occurrence of trouble of either the flow control valve or the cut-off valve.